Add toStructuredText to ConstantExpression and align method names with ConstantValue

tests/compiler/dart2js/const_exp_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'type_test_helper.dart';
 
 test(String constantInitializer, [String expectedOutput]) {
@@ skipping 16 matching lines @@
     const toplevelConstant = 0;
     toplevelFunction() {}
     const constant = $constantInitializer;
 """, expectNoWarningsOrErrors: true).then((env) {
      var element = env.getElement('constant');
      Expect.isNotNull(element, "Element 'constant' not found.");
      var constant = env.compiler.constants.getConstantForVariable(element);
      var value = env.compiler.constants.getConstantValue(constant);
      Expect.isNotNull(constant,
                       "No constant computed for '$element'.");
-     Expect.equals(expectedOutput, constant.getText(),
-         "Unexpected to string '${constant.getText()}' for constant "
+     Expect.equals(expectedOutput, constant.toDartText(),
+         "Unexpected to string '${constant.toDartText()}' for constant "
          "'$constantInitializer' of value "
-         "${value.toStructuredString()}");
+         "${value.toStructuredText()}");
    });
 }
 
 void main() {
   asyncTest(() => Future.forEach([
     test('null'),
     test('0'),
     test('1.5'),
     test('true'),
     test('false'),
@@ skipping 45 matching lines @@
     test('t ? t : f ? f ? 0 : 1 : 2'),
     test('t ? t ? t : t : t ? t : t'),
     test('t ? (t ? t : t) : (t ? t : t)',
          't ? t ? t : t : t ? t : t'),
     test('const [const <dynamic, dynamic>{0: true, "1": "c" "d"}, '
          'const Class(const Class<dynamic, dynamic>(toplevelConstant))]',
          'const [const {0: true, "1": "cd"}, '
          'const Class(const Class(toplevelConstant))]'),
   ], (f) => f()));
 }